Discriminating apparatus for moving translucent sheets

ABSTRACT

Apparatus for discriminating between moving translucent sheets comprising a sheet transportation means adapted to convey the sheets in spaced relationship along a flow-line, a photoelectric detection device sited on one side of the flow-line, a first differential amplifier having an input in circuit with the said detection device and adapted to produce a first output signal every time the photoelectric device detects attenuation of the light source in excess of a first predetermined value, and a second differential amplifier having an input also in circuit with the said detection device and adapted to produce a second output signal every time the photoelectric device detects attenuation of the light source in excess of a second different predetermined value.

Unlted States Patent 1 1 1 1 3,731,916 Pettet 1 1 May 8, 1973 [54]DISCRIMINATING APPARATUS FOR 3,611,887 10 1971 Shibanuma....

MOVING TRANSLUCENT SHEETS 3,520,404 7/1970 Pine 0 3,186,708 6/1965 Hinz..271/57 [75] Inventor: Ronald SIdney Pettet, Horndean,

England Primary Examiner-Richard E. Aegerter 73 Assignee: De La RueInstruments Limited, Attorney-Baldwin. Wight & Brown London, England[57] ABSTRACT [22] Filed: June 1, 1971 Apparatus for discriminatingbetween moving translulzl] Appl' 148744 cent sheets comprising a sheettransportation means adapted to convey the sheets in spaced relationship[30] F i A li ti P i it D t along a flow-line, a photoelectric detectiondevice I sited on one side of the flow-line, a first differential June1, 1971 Great Britain ..26,940/70 amplifier having an input in circuitwith the Said tection device and adapted to produce a first output [52]US. Cl ..271/57 signal every time the photoelectric device detects [51]IntiCl. .....2B065h 7/0711 tenuation of the light source in excess of afirst [58] d Search predetermined value, and a second differentialamplifier havin an in ut also in circuit with the said detecg P tiondevice and adapted to produce a second output [56] Relerences Cltedsignal every time the photoelectric device detects at- UNITED STATESPATENTS tenuation of the light source in excess of a second differentpredetermined value. 3,578,315 5/1971 Milford ..271/57 3,525,433 8/1970Babb ..209/l1 1.7 17 Claims, 4 Drawing Figures PATENTED W 81973 SHEET 1[IF 2 I PATENTEDHAY' 1913 SHEET 2 BF 2 l or:

OUTPUT F02, w

THICKNESS -0 eEsE'r LATCHE'D GATE PULSE COUNTER cal/NT col/u COUNT A. 5.C.

o/ V5127 a COMMAND GEM- g Yes Na W412 w 05 1/2 29' y COUNT INHIBIT PULSEE" GENL PE L2 2 DISCRIMINATING APPARATUS FOR MOVING TRANSLUCENT SHEETSThis invention relates to apparatus adapted to discriminate betweenlengths and thickness of translucent sheet material travelling along aflow-line at high speed and optionally to count some or all of suchsheets. More especially, the invention relates to such apparatus whichincludes a photoelectric detection means and an associated light sourcedisposed on opposite sides of the flow-line in such a manner that thephotoelectric means is subjected to variations of light intensity duringthe passage of the sheets.

BACKGROUND OF THE INVENTION In many sheet feeding machines, for example,in the case of machines adapted to feed and count treasury notes, it isessential that an exact count is recorded of the number of piecesactually delivered. To this end it is usual to provide means whichensure that multiple or superposed notes are not counted as a singleunit.

A known method of achieving the above requirement is to feed the notesthrough a critical gauging throat which may be arranged to hold backsuperposed notes, or to stop the machine when sheets of abnormalthickness are present, or more preferably to actuate a divertor meanswhich serves to remove multiple notes from the flow-line before they arefed to a counting and delivery stage. An example of the latterarrangement is described in our British Pat. No. 898,715 in which thegauging throat comprises a nip of a critical dimension formed between apair of driven gauging rollers. It is so arranged that normal thicknessnotes pass through the nip, but multiple or excess thickness notes causea displacement of the axis center of one of the rollers which bringsinto effect a solenoid actuated divertor. Counting is carried out bymeans of an electrical make-andbreak circuit disposed downstream of thedivertor which advances a digital counter in an incremental manner. Suchan arrangement possesses a finite operating speed because of themechanical inertia of the detection means and, furthermore, because ofthe difficulties in differentiating between relatively small differencesof thickness at high speed, considerable constructional problems arecreated. It has been found in practice that the gauging nip must beadjusted within fine limits to promote the desired sensitivity toabnormal thickness and furthermore mechanical wear of certain criticalcomponents necessitates careful maintenance to retain the desired degreeof reliability.

SUMMARY OF THE INVENTION It is one object of the present invention toprovide apparatus for discriminating between sheets of theabove-described kinds without the need for a precise mechanical gaugingthroat.

According to the present invention apparatus for discriminating betweenmoving translucent sheets comprise a sheet transportation means adaptedto convey the sheets in spaced relationship along a flow-line, aphotoelectric detection device sited on one side of the flow-line and anassociated light source sited on the other side of the flow-line, afirst differential amplifier having an input in circuit with the saiddetection device and adapted to produce a first output signal every timethe photoelectric device detects attenuation of the light source inexcess of a first predetermined value, and a second differentialamplifier having an input also in circuit with the said detection deviceand adapted to produce a second output signal every time thephotoelectric device detects attenuation of the light source in excessof a second different predetermined value. Thus in one embodiment of theinvention apparatus for handling sheets of a like-kind may be so adaptedthat the output of the said first differential amplifier controls afirst digital counting circuit to record the total quantity of singlesheets or groups of adhered-together sheets fed along the flow-line, andthat the output of the said second differential amplifier controls asecond digital counting circuit to record solely the number of groups ofadhered-together sheets.

Preferably the apparatus also includes a sheet divertor means disposeddownstream of the said detection device and under the control of thesaid second differential amplifier, the arrangement being such that thedivertor, which is normally inoperative, is moved to an operativeposition every time the said second output signal is created. In aspecific embodiment it may be arranged for a count pulse to be inhibitedfrom the said first counting circuit every time the divertor is moved tothe operative position thereby to record only the passage of singlesheets.

Preferably the apparatus also includes a pulsecounter unit fed from apulse generator via a gate under the control of the output of the firstdifferential amplifier, said unit being capable of ascertaining thelength of a sheet or the overall length of overlapping sheets in thedirection of the flow-line thereby, in conjunction with the output ofthe second differential amplifier, to detect the presence of superposed,partially overlapping or edge-abutting sheets.

Preferably the said pulse generator is driven in unison with the lineartravel of the sheets in the flowline. In a convenient form the pulsegenerator comprises a circumferentially perforated or notched wheelmember positively driven by the drive mechanism of the said sheettransportation means, and adapted to. influence a transducer which maybe of the optical or magnetic kind.

One embodiment of the invention will now be described with reference tothe accompanying drawings of which:

FIG. 1 is a side elevation of a banknote feeding and transportationmechanism showing the basic parts thereof;

FIG. 1a is a fragmentary end view of some of the parts of the mechanismof FIG. 1;

FIG. 2 is a diagram of part of an electrical circuit for performing theoptically discriminating aspect of the invention; and

FIG. 3 is a schematic block plan showing the interconnection of variouselectronic assemblies useful in the performance of the invention.

In FIG. 1, the banknote feeding, transporting and deflecting mechanismsare substantially similar to those disclosed in our British Pat. No.898,715 and comprise essentially an inclined stack support member 1adapted to support a stack of banknotes N on edge. The notes are biassed(by means not shown) towards an oscillatory suction arm 2 provided withan angled suction port 3 which contacts the lower central face of thefrontmost note in the stack and feeds it into the nip of a pair ofprimary feed rollers 4 and 5. The lower roller 5 is provided with acut-away portion 6 to facilitate entry of the leading side edge of thenote. The arm is oscillated by means of a tubular shaft member 7 whichis adapted to be moved in a bi-rotational manner by a cam system drivenin unison with the primary feed rollers 4 and 5; the interior of theshaft 7 is in communication with a source of suction (not shown). Whenthe circumferential surfaces of the feed rollers co-act, the note ispulled from the face of the suction port 3, and is passed between alight source L1 and an associated photoelectric detector PE1, and thenceinto the nips of three pairs of conveying rollers 8,9; 10,1 1; and 12,13respectively. The nips of the primary and conveying rollers collectivelydefine a substantially linear flowline for the notes which terminates atthe arrow 14; delivery means beyond the arrow form no part of thepresent invention.

A note divertor means provided between the conveying rollers 10,11 and12, 13 comprises a plurality of spaced fork members 15 rigidly attachedto a shaft 16 which also includes an actuating lever 17 connected to theslidable armature 18 of a solenoid 19 by means of a connecting link 20.The divertor assembly is biassed to an inoperative position, as shown,by means of a tension spring 21. An auxiliary rubber-surfaced drivenroller 22 is provided above the upper conveying roller 10, and a rejectnote hopper 23 is provided above the rollers 4 and 8 to receive divertednotes. Energization of solenoid 19 causes the divertor assembly to moveto the position indicated by the chain-dotted line, whereupon the tipsof the fork members 15 descend into the flow-line thereby to cause anote or notes to be propelled around 180 of the circumference of theroller 10 and thence into the hopper 23.

To facilitate the passage of notes through the roller system a series ofparallel guide members, one of which is indicated by symbol 24, areprovided below the flowline.

The entire roller system is driven at a constant peripheral speedthrough a gear train generally indicated at 25, by an electric motor 26via a two-stage gear reduction disposed on secondary shafts 27 and 28.The shaft 27 (see also FIG. la) mounts a fixedly attached disc member 29which is provided with an equally pitched circumferential row of holes30. A light source L2 and an associated photoelectric detector PE2 aredisposed in alignment with the holes 30 on the opposite sides of thedisc.

The electronic circuitry of the apparatus will now be described withreference to FIGS. 2 and 3. It should be noted that the two outputs xand y of FIG. 2 are connected respectively to the left-hand side of FIG.3.

The output of the photoelectric detector PEl is fed into a linearamplifier 50 the output of which feeds into two substantially similarnetworks, and thence through respective differential amplifiers topoints x and y. The assembly I serves to provide an output signal at xfor each single note or group of partially or wholly superposed notesthat pass between L] and PEI, and the assembly II (which is of coursepre-set in a different manner), serves to provide an output signal at yfor each multiple thickness note or notes, or each single note having anabnormally high opacity that passes between L1 and PEI.

The output of the linear amplifier 50 passes through a rectifier MRwhich, with a capacitor C1, forms a peak level detection means. When nonote is interposed between L1 and PEI the linear amplifier produces apeak output proportional to the intensity of the light falling on thephotoelectric detector PEI. The rectifier MR permits only the positivepeak to charge the capacitor C1 and effectively prevents reverse flow ofcurrent from C1 when the light falling upon PE] is attenuated by thepresence of a note.

An R/C network comprising C1, R1, RVl and R2 provides a time-constantsufficiently long to prevent any significant discharge of C1 during theperiod when light is attenuated. The arrangement is self-compensatinginsofar that a gradual fall-off of light output created by the ageingeffect of the light source, a deterioration in the sensitivity of thephotoelectric detector or a reduction of gain of the linear amplifier,is compensated for automatically because the charge of Cl will alsoreduce at the same rate.

Having charged the peak level detection means up to the voltage producedby the unattenuated light source, a proportion of it is coupled to theinverting" input of the differential amplifier via the slider of thepotentiometer RVl. The potentiometer thus provides a presetting meansfor adjusting the sensitivity of the assembly. The full output of thelinear amplifier, taken from a point upstream of the rectifier MR, isconnected to the positive non-inverting input of the differentialamplifier. Hence, when the light falling upon the photoelectric detectoris unattenuated the voltage to the non-inverting input provides positivefeed-back so that the output x of the differential amplifier will be amaximum positive. Conversely, when the light level is attenuated, theoutput of the linear amplifier will fall to below the level set on theinverting input with the result that the differential input will beinverted thereby to produce a negative output x from the differentialamplifier. A positive feed-back loop comprising the resistor R4 ensuresthat the output x will be a maximum negative.

The assembly II, which terminates at an output y, is substantiallysimilar to that described above and serves to produce an output pulse ofchanged polarity when the light is attenuated to a greater degree, forexample on the passage of a plurality of adhered-together notes.

The means for discriminating between the length of a single sheet andthe overall length of superposed sheets, as measured in the direction ofthe flow-line, will now be described.

Operation of the roller system of the machine also causes, through themedium of the gearing system, the perforated disc member 29 to rotate,with the result that a succession of electrical pulses is created in thephotoelectric detector PE2. These pulses are fed into a Pulse Counter"via a "Latched Gate" (see FIG. 3) which is opened at the commencement ofevery period of attenuation of PEI, via the electronic assembly I andclosed after a preset number of pulses have been fed into the PulseCounter. The number of pulses fed into the Pulse Counter" is directlyproportional to the angular displacement of the shafts in the drivemechanism, and hence to the length of the sheet being fed.

The Pulse Counter provides outputs in accordance with three differentpreset and ascending numbers of datum counts which permit decisions tobe effected in accordance with the overall length of a sheet or sheetspassing through the optical system. These counts are routed in thefollowing manner:

COUNT A into a Divert Command Generator which also receives signals fromthe electronic assembly II via connection y. The Divert CommandGenerator is provided with two alternative outputs viz, a YES output tothe divertor solenoid 19 and a Count Pulse Generator", or a NO output toa Warning Gererator".

If signals from connections y are received before COUNT A is reached,the YES output is obtained, the diverter solenoid 19 is actuated and anINHIBIT signal is fed to the COUNT PULSE GENERATOR. If COUNT A isexceeded before signals are received from the connection y, the NOoutput is energized and the WARNING GENERATOR" is energized.

COUNT B into the Warning Generator (which also receives signals from thex connection during the entire period of the attenuation of the light ofL1) and thence to a warning device shown diagrammatically at symbol 51.COUNT C into the Count Pulse Generator (which, as mentioned above, alsoreceives INHIBIT signals when the divertor is actuated) and thence intoa digital counter and display unit shown diagrammatically at symbol 52.In addition a RE-SET pulse is fed back to unlatch the LATCHED GATE uponattainment of COUNT C. Actuation of the Apparatus,

including examples of correct and incorrect note feeding.

Energization of the driving motor 26 will cause banknotes to be fedsequentially from the front of the stack N into the roller system alongthe flow-line thereby to attenuate in an intermittent manner the lightfalling upon the photoelectric detector PEI. Pulses will also begenerated by the photoelectric detector PE2 through rotation of the discmember 29.

Case 1 The passage of a normal length single note will record a countingpulse in the counter unit 52 in the following manner:

When the leading edge of the note attenuates the light source a signalfrom the all-thickness assembly I opens the Gate which allows pulsesfrom PE2 to pass into the Pulse Counter. For a note of normal length,the light attenuation signal from x into the WARN- ING GENERATORterminates before COUNT B is reached and thus no warning is generated.When the Pulse Counter reaches "COUNT C" a count pulse is generated anda count of one is registered in the counter unit 52.

Case 2 The passage of two superposed or substantially superposed noteswill be rejected in the following manner:

In a manner similar to that described in (1) above, the Gate passespulses to the Pulse Counter but in this case, a multiple-thickness pulseis generated by the assembly II through the connection y and DivertCommand Generator". If the y signal, which is indicative of thecommencement of the multiple thickness during the passage of the sheets,is created before COUNT A is reached the sheets are capable of diversionand accordingly the Divert Command Generator generates a YES signal andenergizes the solenoid 19 of the divertor thereby to cause thesuperposed notes to be conveyed into the reject note hopper 23. At thesame time the count pulse at Count C is inhibited and no count isrecorded in the counter unit 52. If signal y is created after COUNT A isreached, diversion is not possible and the superposed or substantiallysuperposed sheets are treated as partially overlapping sheets (see Case3 below).

Case 3 A The passage of two partially-overlapping notes will be handledin the following manner:

The cycle of operation is initiated as in Case 2 above but in this caseCount A is reached before the creation of the y signal and the DivertCommand Generator does not energize the solenoid 19 of the divertor.This prevents the mechanism from attempting to divert a note, theleading edge of which has already passed below the tips of the divertorfork members 15. Instead, a NO signal is applied to the WarningGenerator which gives a warning signal and the count pulse is notinhibited.

Case 4 The passage of two edge-abutting notes (viz. with no overlap) ishandled as follows:

The cycle of operation is initiated as in Case 3 above, but in this caseno multiple-thickness is detected. However, when Count B" is reached theoutput of the allthickness assembly I at connection x into the WARN- INGGENERATOR is still maintained thus indicating the presence of anabnormally long article. The Warning Generator initiates a warning andthe Count Pulse Generator is activated as in Case 3.

It will be appreciated that the above described apparatus is capable ofcounting and discriminating translucent sheets at a very high speed, theupper limit of which is dependent only upon the mechanical limitationsof the feeding mechanism. Furthermore, the

photoelectric detection means and associated circuitry isself-compensating and thus after an initial setting-up, is capable ofoperating for long periods without attention.

It will further be appreciated that the complete elimination of the needfor a critical gauging throat to detect multiple-thicknesses hassimplified the mechanical construction of machines of this general typeand avoided the need for frequent adjustment and maintenanee.

What we claim is:

1. Apparatus for discriminating between moving translucent sheetscomprising a sheet transportation means for conveying the sheets inspaced relationship along a flow-line, a single photoelectric detectiondevice positioned on one side of the flow-line and an associated lightsource positioned on the other side of the flow-line, a firstdifferential amplifier means for producing a first output signal everytime the photoelectric detection device detects attenuation of the lightsource in excess of a first predetermined value and having an input incircuit with the said detection device, and a second differentialamplifier means for producing a second output signal every time thephotoelectric detection device detects attenuation of the light sourcein excess of a second different predetermined value greater than saidfirst predetermined value and having an input also in circuit with thesaid detection device.

2. Apparatus as claimed in claim 1 in which the first output signalserves to indicate the passage of (a) each single sheet having anopacity greater than the first predetermined value and less than thesecond predetermined value; (b) each folded single such sheet; (c) eachsingle sheet having an opacity greater than the second predeterminedvalue; and (d) each group of partially or wholly superposed singlesheets having an opacity greater than the second predetermined value,and the second output signal serves to indicate the passage of any sheetin category (b), (c) or (d).

3. Apparatus as claimed in claim 1 together with means for coupling thefirst output signal to a first digital counter for actuating the sameand means for coupling the second output signal to a second digitalcounter for actuating the same.

4. Apparatus as claimed in claim 3 together with means for inhibitingthe first output signal to the first digital counter upon the productionof the second output signal.

5. Apparatus as claimed in claim 1 in which a sheet divertor is providedin the flow-line at a point downstream of the photoelectric detectiondevice, said divertor being normally inoperative with respect to thesheets, but being operable to move into the flow-line every time thesaid second output signal is produced.

6. Apparatus as claimed in claim 5 which additionally includes lengthdiscrimination means for inhibiting actuation of the sheet divertor ifthe length of a sheet, or the overall length of a group of partially orwholly superposed sheets in the direction along the flow-line, exceeds apredetermined dimension.

7. Apparatus as claimed in claim 6 in which the said lengthdiscrimination means comprises a pulse generator driven in unison withthe sheet transportation means, a pulse-counting means, gate-switchingmeans for directing a train of pulses to the pulse-counting means whilsta sheet is present between the light source and the photoelectricdetection device, and a countdiscriminator for activating the sheetdivertor only if less than a predetermined number of pulses are directedinto the pulse-counting means.

8. Apparatus as claimed in claim 7 in which the said pulse generatorcomprises a circumferentially perforated wheel positively driven by thesheet transportation means, a second light source disposed on one sideof the wheel in alignment with the said perforations, a secondphotoelectric-detection means disposed on the other side of the wheel inalignment with the said perforations and operable upon rotation of thewheel to feed a train of pulses into the said gate-switching means.

9. Apparatus as claimed in claim 5 so constructed that the creation ofthe said first output signal, and the termination thereof before theattainment of the second datum count and the creation of the said secondoutput signal before the attainment of the first datum count indicatesthe passage ofa plurality of overlapping sheets and further adapted tocause said divertor to move into the flow-line upon the passage of suchsheets.

10. Apparatus as claimed in claim 5 so constructed that the creation ofthe said first output signal and the termination thereof after theattainment of the second datum count and the creation of the said secondoutput signal after the attainment of the first datum count indicatesthe passage of a plurality of overlapping sheets, and further adapted toinhibit the movement of said divertor into the flow-line and to actuatea warning means.

11. Apparatus as claimed in claim 1 in which each differential amplifiermeans is provided with manually adjustable means adapted to permit theinitial setting up of its output level.

12. Apparatus as claimed in claim 11 in which said input to eachdifferential amplifier means includes one input dependent upon theeffect of attenuated light upon said photo-electric device and a secondinput dependent upon the effect of direct light upon said photoelectricdevice, said second input including said adjustable means.

13. Apparatus as claimed in claim 1 which additionally includes lengthdiscriminator means including a pulse generator driven in unison withthe sheet transportation means, a pulse-counting means, gate switchingmeans for directing a train of pulses to the pulse-counting means duringproduction of said first output signal, the pulse counter being operableto produce successive first and second sheet length-indicative datumcounts.

14. Apparatus as claimed in claim 13 so constructed that the creation ofthe first output signal and the termination thereof after the attainmentof the first datum count, but before the attainment of the second datumcount, indicates the passage of a sheet of predetermined opacity andlength.

15. Apparatus as claimed in claim 13 so constructed that the creation ofthe first output signal and the termination thereof after the attainmentof the second datum count indicates the passage of at least one sheethaving an overall length in excess of a predetermined length.

16. Apparatus as claimed in claim 15 including warning means operableupon the passage of one or more sheets having an overall length inexcess of a predetermined length.

17. Apparatus as claimed in claim 13 in which the said pulse generatorcomprises a circumferentially perforated wheel positively driven by thesheet transportation means, a light source disposed on one side of thewheel in alignment with the said perforations, a photoelectric detectionmeans disposed on the other side of the wheel in alignment with the saidperforations and adapted upon rotation of the wheel to feed a train ofpulses into the said gate-switching means.

1. Apparatus for discriminating between moving translucent sheetscomprising a sheet transportation means for conveying the sheets inspaced relationship along a flow-line, a single photoelectric detectiondevice positioned on one side of the flow-line and an associated lightsource positioned on the other side of the flow-line, a firstdifferential amplifier means for producing a first output signal everytime the photoelectric detection device detects attenuation of the lightsource in excess of a first predetermined value and having an input incircuit with the said detection device, and a second differentialamplifier means for producing a second output signal every time thephotoelectric detection device detects attenuation of the light sourcein excess of a second different predetermined value greater than saidfirst predetermined value and having an input also in circuit with thesaid detection device.
 2. Apparatus as claimed in claim 1 in which thefirst output signal serves to indicate the passage of (a) each singlesheet having an opacity greater than the first predetermined value andless than the second predetermined value; (b) each folded single suchsheet; (c) each single sheet having an opacity greater than the secondpredetermined value; and (d) each group of partially or whollysuperposed single sheets having an opacity greater than the secondpredetermined value, and the second output signal serves to indicate thepassage of any sheet in category (b), (c) or (d).
 3. Apparatus asclaimed in claim 1 together with means for coupling the first outputsignal to a first digital counter for actuating the same and means forcoupling the second output signal to a second digital counter foractuating the same.
 4. Apparatus as claimed in claim 3 together withmeans for inhibiting the first output signal to the first digitalcounter upon the production of the second output signal.
 5. Apparatus asclaimed in claim 1 in which a sheet divertor is provided in theflow-line at a point downstream of the photoelectric detection device,said divertor being normally inoperative with respect to the sheets, butbeing operable to move into the flow-line every time the said secondoutput signal is produced.
 6. Apparatus as claimed in claim 5 whichadditionally includes length discrimination means for inhibitingactuation of the sheet divertor if the length of a sheet, or the overalllength of a group of partially or wholly superposed sheets in thedirection along the flow-line, exceeds a predetermined dimension. 7.Apparatus as claimed in claim 6 in which the said length discriminationmeans comprises a pulse generator driven in unison with the sheettransportation means, a pulse-counting means, gate-switching means fordirecting a train of pulses to the pulse-counting means whilst a sheetis present between the light source and the photoelectric detectiondevice, and a count-discriminator for activating the sheet divertor onlyif less than a predetermined number of pulses are directed into thepulse-counting means.
 8. Apparatus as claimed in claim 7 in which thesaid pulse generator comprises a circumferentially perforated wheelpositiVely driven by the sheet transportation means, a second lightsource disposed on one side of the wheel in alignment with the saidperforations, a second photoelectric-detection means disposed on theother side of the wheel in alignment with the said perforations andoperable upon rotation of the wheel to feed a train of pulses into thesaid gate-switching means.
 9. Apparatus as claimed in claim 5 soconstructed that the creation of the said first output signal, and thetermination thereof before the attainment of the second datum count andthe creation of the said second output signal before the attainment ofthe first datum count indicates the passage of a plurality ofoverlapping sheets and further adapted to cause said divertor to moveinto the flow-line upon the passage of such sheets.
 10. Apparatus asclaimed in claim 5 so constructed that the creation of the said firstoutput signal and the termination thereof after the attainment of thesecond datum count and the creation of the said second output signalafter the attainment of the first datum count indicates the passage of aplurality of overlapping sheets, and further adapted to inhibit themovement of said divertor into the flow-line and to actuate a warningmeans.
 11. Apparatus as claimed in claim 1 in which each differentialamplifier means is provided with manually adjustable means adapted topermit the initial setting up of its output level.
 12. Apparatus asclaimed in claim 11 in which said input to each differential amplifiermeans includes one input dependent upon the effect of attenuated lightupon said photo-electric device and a second input dependent upon theeffect of direct light upon said photoelectric device, said second inputincluding said adjustable means.
 13. Apparatus as claimed in claim 1which additionally includes length discriminator means including a pulsegenerator driven in unison with the sheet transportation means, apulse-counting means, gate switching means for directing a train ofpulses to the pulse-counting means during production of said firstoutput signal, the pulse counter being operable to produce successivefirst and second sheet length-indicative datum counts.
 14. Apparatus asclaimed in claim 13 so constructed that the creation of the first outputsignal and the termination thereof after the attainment of the firstdatum count, but before the attainment of the second datum count,indicates the passage of a sheet of predetermined opacity and length.15. Apparatus as claimed in claim 13 so constructed that the creation ofthe first output signal and the termination thereof after the attainmentof the second datum count indicates the passage of at least one sheethaving an overall length in excess of a predetermined length. 16.Apparatus as claimed in claim 15 including warning means operable uponthe passage of one or more sheets having an overall length in excess ofa predetermined length.
 17. Apparatus as claimed in claim 13 in whichthe said pulse generator comprises a circumferentially perforated wheelpositively driven by the sheet transportation means, a light sourcedisposed on one side of the wheel in alignment with the saidperforations, a photoelectric detection means disposed on the other sideof the wheel in alignment with the said perforations and adapted uponrotation of the wheel to feed a train of pulses into the saidgate-switching means.